“ In determining that the acquisition of the assets of Herring-Hall-Marvin Safe Company was not a violation of § 7, the District Court acted upon its findings that 'HHM was hopelessly insolvent and faced with imminent receivership' and that 'Diebold was the only bona fide prospective purchaser for HHM's business.' The latter finding represents at least in part the resolution of a head-on factual controversy as revealed by the materials before the District Court of whether other offers for HHM's assets or business were actually made. ”